There are many situations where there is a need to support a planar structure at variable distances in height above the ground. One such application is that of a storage tank that includes a floating roof disposed therein. This example use case can be best described with reference to FIG. 1. In FIG. 1, a floating roof structure 107 is typically used where a storage tank 205 is used to store a liquid, for example, jet fuel, gasoline, diesel, sour water and crude oil. It should be appreciated that these are merely examples of the type of liquids that can be stored in a tank.
It is common place for such a storage tank to include a floating roof structure for environmental protection purposes. Floating roofs substantially reduce the emission of flammable and/or hazardous vapor to the environment. It should be appreciated that such a floating roof structure “floats” on top of the liquid product stored in the storage tank. As the level of the liquid product stored in the tank fluctuates, so does the height of the floating roof structure relative to a floor included in such a tank.
Government regulations mandate that such storage tanks be inspected every ten years. Such an inspection is generally required to meet the requirements of API 653. The regulations require that all above ground storage tanks are to be inspected and repaired to API 653 standards in order to verify the structural integrity of the tank shell, the integrity of the floating roof vapor control, and the tank floor. One aim of such inspections is to detect and remedy any seepage of hazardous, toxic or flammable liquids into the ground.
Undetected seepage is likely to cause environmental impact with wide reaching consequences, such as pollution of water tables. For example, more frequent inspection could have prevented a recent environmental crisis in West Virginia where a hazardous chemical leaked from a storage tank and contaminated the water table. Because such inspections are known to reveal the type and extent of repairs needed to remedy leaks and other environmental cataclysms, it is unlikely that any of these inspection requirements will ever be abated. There are also occasions when the storage tank must be cleaned in preparation for storing a different liquid product or a different class of a liquid product relative to a former substance previously stored in the tank. The floating roof must be held above the floor of the storage tank so that personnel can freely and safely conduct themselves during all such inspection, repair and cleaning activities.
FIG. 1 is a pictorial representation of a prior art apparatus for supporting a floating roof when a storage tank is devoid of liquid content. For years and years and years, the process of supporting a floating roof in the absence of a liquid product has been accomplished using substantially similar methods, each of which rely on the use of substantially identical support apparatus. As can be seen, the prior art has thus far relied on a basic support method using a “cribbing stack,” also known as a “vertical load backup.”
A cribbing stack 103 is typically made up of alternating layers of wood members, wherein each wood member from a preceding layer is set orthogonal to a subsequent layer. Hence, the height of the cribbing stack could be adjusted by simply stacking up more of such alternating layers of wooden members. Up until now, this prior art technique has been used without much deviation from this basic concept, that being the use of alternating layers of wooden members. It should be noted that these wooden members are somewhat akin to common railroad ties that are readily available throughout the world.
FIG. 1 also depicts one grave disadvantage associated with the use of a wooden, layered cribbing stack. It is well settled that a floating roof may exhibit rotational forces 109. When the floating roof is first lowered and substantially all product is removed from the tank, a collection of “legs,” each of which penetrates the floating roof, are used to support the floating roof. These legs are very susceptible to horizontal forces that each leg experiences when the roof begins to rotate. This is true regardless of whether the storage tank is empty or of it has liquid content.
Wind can induce such rotational movement of the floating roof. There are methods to retard such rotational movement, but these methods often fail. One such method is based on the use of “anti-rotation wedges.” These wedges are, by their very name, disposed between an outer perimeter of the floating roof and an internal wall of the storage tank. Such anti-rotational wedges are scarcely effective in the face of severe rotational movement of the floating roof.
It is when the floating roof exhibits rotational movement that personnel working under the floating roof situated in a storage tank are most vulnerable to injury and death. When a floating roof begins to rotate, it begins to apply a moment force on each leg. As the legs begin to fail, the plurality of wooden cribbing stacks are intended to support the floating roof at some minimum height necessary to keep all personnel safe. Because the layers of a wooden cribbing stack are not fastened to each other, the cribbing stack simply falls apart when these horizontal forces go unopposed. The upper layers of the cribbing stack, from a force perspective, simply shear away from the lower layers of the cribbing stack. This, of course, results in the type of total failure of the support structure that has cost many lives and has resulted in extensive collateral, material damage and environmental impact.
There are many serious environmental issues associated with the use of a wooden cribbing stack. It should be appreciated that the product ordinarily stored in a storage tank is a liquid and such liquids are typically hazardous materials. Such hazardous material may include petro chemical products, crude oil, flammable liquids and many other forms of extremely hazardous materials. Residual product in the storage tank will ordinarily permeate the wooden members. Hence, such contaminated wooden members cannot be reused and must be discarded as hazardous waste. Each time a wooden member is discarded, new lumber must be used at the cost of many trees, harvested from our forests, further impacting global warming and greenhouse gas effects.